1. Field of the Invention
The present invention relates to a measurement apparatus and a measurement method for measuring a light intensity distribution, an exposure apparatus including the measurement apparatus, and a device manufacturing method.
2. Description of the Related Art
Conventionally, in order to evaluate the performance of an optical system in a state where the optical system is installed in a semiconductor exposure apparatus, a mask pattern was exposed onto a wafer on which a resist is applied. This evaluation method needed to develop the resist after the exposure and to measure the resist image formed by the development using a SEM (Scanning Electron Microscope) or the like. Thus, since this evaluation method needs a plurality of processes such as a resist application, a development, and a measurement, a lot of time and cost are necessary per one evaluation.
Therefore, other than the above evaluation method, without performing an actual exposure, a method for imaging a mask pattern or a pattern for measurement in the air corresponding to a wafer plane and directly measuring the light intensity distribution by a measuring instrument (hereinafter referred to as an “aerial image measurement method”) has been performed. As one example of this method, a slit scan method which scans a slit having a width narrower than a wavelength and measures light that has been transmitted through the slit by a light receiving element (For example, see “W. N. Partlo, C. H. Fields and W. G. Oldham, J. Vac. Sci. Technol. B, Vol. 11, pp. 2686-2691”).
In the slit scan method, a slit formed on a light shielding film is used, and an aerial image having a periodic intensity distribution is formed by illuminating a line-and-space pattern (hereinafter referred to as an “L/S pattern”) to form an image of this, for example. Partial light of the formed aerial image is transmitted through the slit, and the transmitted light that has been transmitted through the slit is irradiated on a light receiving element after being transmitted through a transmissive substrate which supports the light shielding film. The light that has been irradiated on the light receiving element is photo-electrically converted and is outputted as a slit signal.
A measurement sensor which is constituted by the light shielding film, the transmissive substrate, and the light receiving element is scanned at a stage in an X-axis direction, and the slit signal is monitored per a scan step. The aerial image is measured by scanning this slit and using the monitored signal (hereinafter referred to as a “slit scan signal”).
However, in the conventional slit scan method, in the case where the period of the intensity distribution fluctuation of the aerial image is shortened, if a longitudinal direction of the slit differs from a direction parallel to the L/S pattern of the aerial image, the modulation degree of the slit scan signal is extremely reduced.
The aerial image is, for example, an L/S pattern which is formed in parallel to a Y-axis direction, and in this case, the light intensity distribution is modulated in an X-axis direction at a half period HP. A position difference is generated between the direction parallel to the L/S pattern of the aerial image and the longitudinal direction of the slit formed on the light shielding film in an XY plane by an angle θ in a rotational direction. Ideally, if the angle θ is equal to zero, a slit scan signal with high modulation degree can be obtained. However, actually, the angle θ is not equal to zero because of an alignment error or the like.
When the slit scan is performed in the circumstances, the modulation degree of the slit scan signal is reduced compared to that of the aerial image. Furthermore, when the slit is displaced up to such an angle that it strides across the L/S pattern period of the aerial image, the light intensity irradiated on the slit does not change even if the slit is scanned. Therefore, since there is substantially no modulation degree of the slit scan signal, the measurement is impossible. When the angle between the slit and the L/S pattern is defined as a critical angle θc, it is represented by expression 1.θc=arcsin(2HP/SL)  (1)
In expression 1, SL is a length of a slit in a longitudinal direction (a slit length) and HP is a half period of the light intensity distribution fluctuation of an aerial image. If the angle between the slit and the L/S pattern is smaller than the critical angle θc, the light intensity irradiated on the slit is modulated by scanning the slit.
FIG. 21 shows a relationship between the critical angle θc and the slit length SL. In FIG. 21, a vertical axis indicates the critical angle θc, and a lateral axis indicates the slit length SL. As shown in FIG. 21, when the slit length SL is around 50 μm, the critical angle θc is around 14 mrad in the case where HP is 200 nm. However, when HP is 45 nm, the critical angle θc is reduced to around 2 mrad.
Thus, if HP is reduced, the critical angle θc is reduced and the latitude of the position displacement is lowered. Therefore, in order to obtain the slit scan signal with high modulation degree, such an alignment with high accuracy that makes the angle θ smaller than at least the critical angle θc is required.
Conventionally, for example, a method for scanning a slit while an angle of the slit is changed and adjusting an alignment so that the modulation degree of the scan signal is the highest has been used. However, this method needed a lot of alignment time and the alignment accuracy was reduced because the change of the modulation degree was reduced when the modulation degree was high to some extent.